1. Field of the Invention
The invention relates to piezoelectric sensors.
2. Description of the Related Art
A piezoelectric sensor is known from German Patent 28 431 938. In this sensor the charge transfer of a piezoelectric film, which is converted into the desired measurement signal via an external electronic evaluation unit, is used as the measurement value. These sensors have the disadvantage that the length of the paths to the electronic evaluation unit are limited and a further component must be arranged remotely with the electronic evaluation unit.
Sensors of the above-mentioned type are known in addition as impact sensors having a seismic mass, the seismic mass being pressed against the piezoelectric layer as a result of an impact and the acceleration caused thereby. This pressure on the piezoelectric layer causes, in turn, a charge transfer which can be picked up and evaluated by means of contacts on each side of the surface of the piezoelectric layer. A sensor of this type is described by Gevatter in xe2x80x9cHandbuch der Mess- und Automatisierungstechnikxe2x80x9d, VDI Verlag 1998.
These sensors with a seismic mass have, in addition to the disadvantages of the above-mentioned film-type sensors, the disadvantage that the seismic mass must be excited. Furthermore, because of the movable guidance system of the seismic mass, a comparatively complex and costly mechanism must be provided which, because of the moving parts, leads to high production costs and, in addition, to a higher risk of failure. Finally, construction of very sensitive sensors is possible only at disproportionate cost, as measurement is only possible if the seismic mass is excited by the weak signal. For this reason a sensor of this kind can hardly be used, for example, as a vibration meter.
Hence, those skilled in the art have recognized a need for a sensor which can be manufactured at low cost, is easily installed and can measure economically and in a simple manner even weak signals, such as vibrations or material deformations, by utilizing the piezoelectric effect. The invention fulfills these needs and others.
Briefly, and in general terms, the invention is directed to a piezoelectric sensor including a carrier, a piezoelectric measurement sensing element arranged on the carrier, a covering layer covering the measurement sensing element and an electronic evaluation unit. The measurement sensing element is formed by a piezoelectric layer. The carrier has a first contact layer electrically connected to the piezoelectric layer, the covering layer has a second contact layer electrically connected to the piezoelectric layer and the electronic evaluation unit is able to determine a mechanical loading of the piezoelectric layer by evaluating the difference of electrical potential between the first contact layer and the second contact layer.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings which illustrate by way of example the features of the invention.